Add extra check to server migration complete test

The change [1] modified the share server migration complete
operation behavior in Manila.

In the previous behavior there was a chance that manila would
keep the inactive source share server after a successful
migration.

Such behavior is not possible anymore after this change was
merged, since Manila started to expect that the source share
servers were deleted within the successful completion of a share
server migration.

This change introduces a new waiter to make sure that the source
share server is actually being deleted from manila after the
migration complete phase.

[1] https://review.opendev.org/c/openstack/manila/+/803623

Change-Id: Ibb092111032241a71763da0c7fa40470a3cd95ae
diff --git a/manila_tempest_tests/tests/api/admin/test_share_servers_migration.py b/manila_tempest_tests/tests/api/admin/test_share_servers_migration.py
index 98150b8..f74e043 100644
--- a/manila_tempest_tests/tests/api/admin/test_share_servers_migration.py
+++ b/manila_tempest_tests/tests/api/admin/test_share_servers_migration.py
@@ -388,6 +388,8 @@
         self._validate_share_server_migration_complete(
             share, dest_host, dest_server_id, snapshot_id=snapshot_id,
             share_network_id=dest_share_network_id)
+        self.admin_shares_client.wait_for_resource_deletion(
+            server_id=src_server_id)
 
     @decorators.idempotent_id('52e154eb-2d39-45af-b5c1-49ea569ab804')
     @tc.attr(base.TAG_POSITIVE, base.TAG_BACKEND)